A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weak or killed forms of the microbe, its toxins or one of its surface proteins. Vaccines work by stimulating our immune system to produce antibodies- substances produced by the body to fight disease, without actually infecting us with the disease. They instigate the immune system to produce its own antibodies, as though the body has been infected with a disease which is called ""active immunity"". Vaccination is the injection of a killed or weakened organism that produces immunity in the body against that organism. Vaccines save lives by preventing outbreaks of disease and protecting those who cannot be vaccinated.
There are two basic types of vaccines: live attenuated and in-activated. The characteristics of live and inactivated vaccines are different as live attenuated vaccines are produced by modifying a disease-producing (“wild”) virus or bacteria in a laboratory. The resulting vaccine organism retains the ability to grow and produce immunity, but usually does not cause illness. Live attenuated vaccines include live viruses and live bacteria. On the other hand In-activated vaccines can be composed of either whole viruses or bacteria, or fractions of either:
• Fractional vaccines are either protein-based or polysaccharide-based.
• Protein-based vaccines include toxoids (inactivated bacterial toxin), and subunit or subvirion products.
• Most polysaccharide-based vaccines are composed of pure cell-wall polysaccharide from bacteria while conjugate polysaccharide vaccines are those in which the polysaccharide is chemically linked to a protein and this linkage makes the polysaccharide a more effective vaccine.